Basically when you make decisions with your node you're actively taking a share in the collective security of Bitcoin. If you use a custodian, you delegate this to someone else; the node embodies your participation.

Even when you use a wallet with your own keys that uses BIP-157 (wasabi or LND with Neutrino) you depend on other people offering services that they don't necessarily need to offer publicly, whereas if you run a full p2p node, you plug into a necessary protocol that no one's node can function without and thus is much harder to totally censor. You basically are in full control and with that, add yourself to the club of people making decisions; no matter how small. Every bit helps.

The most compelling argumentation for this that I have seen comes from Eric Voskuil, here:

Both and add help in decentralization. While you have a valid full copy of the timechain you are helping others nodes to validate themselves.

You don’t need third parties to do and verify anything at blockchain.